EP0008943A1

EP0008943A1 - An improved absolute, fluid filter

Info

Publication number: EP0008943A1
Application number: EP79301828A
Authority: EP
Inventors: Allan Henry Meny; Dennis Lucien Palmer
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-09-06
Filing date: 1979-09-05
Publication date: 1980-03-19
Also published as: DE2966224D1; EP0008943B1; JPS5539293A; US4227899A; CA1135637A

Abstract

The novel filter, according to an embodiment thereof, comprises a filter matrix formed into a specific geometry and made of glass, metal, mineral and/or cellulostic fibers, or fibers or fiber-agglomerates moulded from thermoplastic, thermoset plastic, elastoplasts, or related synthetic polymer materials, which have been coated, impregnated, or compounded, prior to forming, with a series of substances in order that the filter may be easily cleaned and reused, as well as protected against corrosive action by the fluid media, or to render substances in the filtration fluid media chemically unreactive, or to change the chemical state or structure of matter in the fluid media. The coating compounds, filler compounds, or impregnants, depending upon which thereof are employed, change the physical properties of the filter matrix and alters impaction-retention of filtered particles, and/or alters the electrostatic attraction of the matrix for matter in the fluid media, and<sub>/</sub>or imparts corrosion protection to the filter matrix, and or renders matter in the fluid media chemically unreactive by changing its reactivity characteristics or chemical structure upon contact with the matrix.

Description

This invention pertains to fluid filters for removing components such as particulate matter from gas streams, and in particular to an improved fluid filter of the absolute type.
Absolute filters, especially HEPA filters, are of uncommon efficiency for removing components such as particulate materials, particulate-aerosol combinants, sublimated solids, vapors, bacteria and virus, some real gases, and the like, from gas streams. However, filters of this type are uncommonly expensive and this fact, together with the impossibility of cleaning such filters for reuse, makes them economically unattractive. Such filters experience an overly efficient particulate impaction-retention, electrostatic attraction, and adhesion to the filter matrix that causes the filter to build up particulate matter, "blinding" the filter system, thus causing a severe gas stream pressure drop and loss of gas media through-put. If mechanical energy is added to the filter system to free the matrix of accumulated particulates, the energy has to be of such magnitude that it will lead to physical damage of the filter material or the structure of the filter matrix. In most cases, without damaging the filter, it cannot be rendered reusable.
It is an object of this invention to set forth an improved absolute, fluid filter which has been rendered easily and non-destructively cleanable and, hence, reusable.
It is particularly an object of this invention to set forth an improved absolute, fluid filter, for removing components such as particulate materials, particulate-aerosol combinants, sublimated solids, and the like, from gas streams, said filter having a matrix formed from a material taken from a group consisting of treated cellulose, glass, metal, and mineral fibers, fiber laminates, and synthetic polymer fibers which have been treated, filled or the like, wherein the improvement comprises means mated to said matrix material for facilitating release of said stream components from said filter, to render said filter easily and non-destructively cleanable, as aforesaid, whereby said filter, upon having been cleaned pursuant to a blinded condition thereof, manifests a change in pressure drop across the filter face which manifests restoration to fifty percent of the used or blinded condition pressure drop. For example, if the filter has a pressure drop of 1.5 inches of water at 1000 cfm of air at STP across a filter with a face area of 500 in.² and, with use, develops a pressure drop of 5 to 15 inches of water at the same condition and blinds, the filter should have to be cleaned to fifty percent of the blinded condition pressure drop, i.e. 2.5 or 7.5 inches of water, for the filter to be considered as having been rendered reusable. It is the aforesaid object of this invention, then, to disclose an improved absolute, fluid filter which can be rendered reusable.
It is another object of this invention to disclose an improved absolute, fluid filter, as aforesaid, wherein said mated means further comprises means for selectively altering physical properties of said matrix material to reduce impaction-retention of said stream components captured by said matrix material, while maintaining filtration efficiency of said filter, and rendering said filter easily and non-destructively cleanable and realizing said pressure condition thereacross as noted.
It is another object of this invention to teach an improved absolute, fluid filter, of the type noted, in which said mated means further comprises means for reducing electrostatic attraction of said matrix material for stream components captured by said matrix material.
It is another object of this invention to set forth an improved absolute, fluid filter, of the type noted, in which said mated means further comprises means for increasing diffusion capacity of said matrix material while maintaining in the latter accessible apertures or paths from where stream components captured by said matrix material can be easily removed without damage to the matrix system and, physically, to bring said filter to said fifty percent (approximately) pressure condition.
It is yet another object of this invention to disclose an improved absolute, fluid filter, according to the foregoing, in which said mated means further comprises means for protecting said matrix material from corrosive action of said stream components, or from damage arising from gross impingement, abrasion, or erosion.
It is still a further object of this invention to teach an improved absolute, fluid filter, as noted in the foregoing, in which said mated means further comprises means for rendering said stream components unreactive, non-carcinogenic, or likewise chemically stable, due to reaction of said components with said mated means, wherein said mated means comprises means for functioning as a gettering agent, and wherein said mated means comprises means for altering the chemical structure or chemical state, functionality or reactivity of the stream components.
Further objects of this invention, as well as the novel features thereof, will become more apparent by reference to the following description.
According to the present invention, there is provided an absolute, fluid filter, for removing components such as particulate materials, particulate-aerosol combinants, sublimated solids, and the like, from gas streams, said filter having a matrix formed from one material taken from a group of materials consisting of treated paper, glass, and mineral fibers, ceramic fibers, polymeric fibers, and cellulostic fibers, characterised by: means coating said matrix material, and hardening, strengthening and toughening said matrix material, for enhancing components release from said filter, whereby said filter is rendered cleanable and reusable.
The invention also provides an absolute, fluid filter, for removing components such as particulate elements, particulate-aerosol combinants, sublimated solids, and the like, from gas streams, said filter having a matrix formed from one material taken from a group of materials consisting of treated paper, glass, and mineral fibers, ceramic fibers, polymeric fibers, and cellulostic fibers, characterised by:

means mated to said matrix material, and hardening, strengthening and toughening said matrix material for facilitating release of said stream components from said filter, to render said filter easily and non-destructively cleanable, whereby said filter, upon having been cleaned pursuant to a blinded condition thereof, manifests a fluid pressure thereacross of approximately fifty percent of such pressure as obtains in such filter in an original, unused condition.

Fluid filters, especially absolute filters of the HEPA type, defined as superinterception apparatus, and so-called "absolute" because of their extremely high retention characteristics and their ability to remove the very smallest particles and the like from gas streams, manifest both impaction and diffusion. In impaction, the to-be-filtered-out components collide with filter matrix fibers and are held -- particularly because the fiber surfaces are relatively light and soft, and they readily accept the components. In diffusion, the components in the gas stream migrate from a region of high component concentration to the regions of lower concentrations, and "diffuse" from the latter to the former to accumulate on the fibers. Now, this phenomena, taken with the electrostatic attraction of the fibers for the components, causes the components to cling tenaciously to the fibers. The impaction- retained components tend to be the larger thereof, whereas the diffused and electrostatically-held components are typically the smaller.
As noted previously, to effect release of the captured or entrapped components, it is the customary practice to add mechanical energy to the filter system. To release the compoments, a sufficient amount of energy must be added to overcome the combined impaction-retention/diffusion/ electrostatic forces. Untreated filters must be energized with mechanical forces that will exceed the strength of the fibers of the matrix, or the fiber-agglomerates, and the filter itself, causing destruction of the filter system. This being the state-of-the-art condition for filter systems, it is our invention to coat, formulate constituents into, or impregnate the filter matrix material with one or more materials which impart singular component release, favorably alter the physical properties of the matrix material fibers, positively control the electrostatic forces of attraction for the gas stream components, and accommodate the egress of captured components by providing accessible apertures or paths from whence stream components can be easily removed. In addition, the coating materials, formulated constituents, or impregnants can simultaneously provide corrosion protection to the filter matrix material to render the gas stream components harmless, or simultaneously getter certain carcinogenics or chemically unstable substances by altering their chemical reactivity or by changing their chemical structure.
According to one embodiment of the invention, a coating material comprising a mixture of graphitic powders of varying particle-size distributions in various thermoplastic and thermoset plastic matrix systems is used. This material is available commercially, under the name EPIS coating, from Epis Corporation, 70 Oakwood Drive, Glastonbury, Connecticut 06033. The coating is pressure sprayed onto one side of the filter media or matrix and drawn by vacuum on the side opposite. The coating is then allowed to reside in the filter matrix, with the spray turned off and with the vacuum on, for a period of time ranging from one to sixteen hours. The coating may be annealed or sintered after the aforesaid fluidizing treatment. According to one of our curing techniques, the matrix is air dried with 250°F air venting to the atmosphere, for from one to ten hours. Next, the heat is raised to 450⁰F, this being held for from one to sixteen hours. Finally, the filter matrix is allowed to cool to room temperature. The coating finishes as a homogeneous, hard film on the fibers of the filter matrix of from 10 to 1000 microns thickness, approximately.
The foregoing treatment reduces the resistivity of the filter matrix media from 10^-10 to 10^-13 ohm-cm (dry) to 10^-4 to 10^-1 ohm-cm depending on the composition of the coating and the thickness thereof. Coatings of this type, when formulated and applied as noted, also impart unusual corrosion resistance to the filter media because of the coating's ability to resist oxidation attack, organic- vapor solvent stress, corrosion cracking, caustic attack, inorganic gaseous attack, or solvation/dissolution attack from chlorinated hydrocarbons, or halogenated acids such as hydrofluoric acid (or hydrogen fluoride). Coating formulations which are useful for realizing the objects of this invention are set out in Table 1.
Alternatively, the foregoing coating compositions, which are not deemed to be exhaustive, may be filled with various agents to improve the performance of the filter matrix. In Table 2 which follows, preferred fillers for the coating compositions of Table 1 are listed. The compositions of Table 1, or those with the fillers of Table 2, may be used on the filter matrix fibers, or they may be impregnated into the matrix fibers; as to the latter, more follows in the ensuing text.
Since most coatings or impregnant must be processed onto or into the filter media, a vehicle, such as a solvent, must be used to allow the coating to form a homogeneous film or to penetrate the filter media fibers. Table 3 lists preferred vehicles.
Accordingly, a typical coating formulation containing a "binder" (from Table 1), a vehicle (i.e. Table 3), or a filler agent (from Table 2) may be applied to, or impregnated into matrix fibers, to impart the inventive properties thereto, and yield the improved filter, and such typical formulations are set out in Table 4.
Formulation numbers 1 through 5 are intended for purpose-α-____ of accommodating gas stream component release, as priorly described. Formulation number 2 and other polytetrafluoroethylene (Table 1) and polyfluoroethylene propylene unfilled coatings impart "self-cleaning" and corrosion protection, while formulation numbers 6 through 10, as well as graphite intercalates of boron trifluoride act as gettering agents (e.g. polymerization of polyvinyl chloride monomer to a polymerized state to remove the monomer from a process gas stream).
The media of the filter matrix to be coated can be made from treated cellulosics, glass, pyroceramic or ceramic fibers, minerals of various types, metals or preoxydized or calorized or plated metals, polymeric materials, and polymer composites, as described previously.
In our preferred embodiment, the matrix is prepared from a filter media material comprising a compound of polyethylene and ketjenblack-EC (the trademark for an electrical conductive carbon black manufactured by Noury Chemical Company, New York). Typically, the constituent elements are cold blended and extruded into fibers, or formed by explosive reticulation techniques. The extruded or otherwise formed fibers are then woven, compressed or hand layed-up into a desired filter configuration. Simply by adjusting the amount of fiber "filler", i.e. the Ketjenblack-EC, it is possible to alter the component (i.e. to-be-filtered-out particles, etc.)-to-media adhesion. That is, differences in the net component (particle)-media volume resistivity, and the net overall volume resistivity of the media surface and/or the fibers, to determine the amount of energy which will be required to break or neutralize the forces of attraction between the components (particles) and media, can be effected by the filler content. Ketjenblack-EC is but one filler which may be used for this purpose, and is mentioned, here, only for exemplary purposes. Other suitable fillers are listed in Table 2.
As noted priorly, various polymeric materials may be used as filter media fibers or material. Table 5 lists polymeric materials that may act as filter media by matrices for which other and various filters are added to alter the properties of the media.
These polymeric materials may also contain filler agents which will modify their physical properties in such a fashion as to control gas stream component impaction-retention, modify surface specific resistivity, and provide a relatively easy means for component release from the filter matrix with the introduction of mechanical, or other, energy forms. Filler agents for the listed polymeric materials, and concentration ranges therefor, are listed in Table 6.
While we have described our invention in connection with specific embodiments thereof, it is to be clearly understood that this is done only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the appended claims.
The invention teaches an improvement of existing components known as HEPA filters, in which the latter is defined as a particular filter system with tested particulate removal of 99.97% of all particles down to 0.3 microns, and where the filtration mechanism is any one or combination of the following: entrainment by impaction retention, impingement, and/or electrostatic attraction. It is a known engineering, and industrial, practical fact that HEPA filters, i.e. absolute filters, will eventually become blinded or inoperative, where blinding is defined as a pressure drop of 5 to 15 inches of water with an air flow of 1000 cfm over a HEPA face area of 500 in.². With a pressure drop from initial unrestricted flow, the filter will block an air stream, cause abnormal resistance and, if the air pressure is maintained, the filter media will rupture. At this point, no means can be used to reclaim the filter because of irreversible damage having occurred to the filter media. The invention discloses an improved HEPA or absolute filter which is rendered reusable, after blinding thereof, by the coating or impregnation of the filter media with a series of specific coating formulations or by fabricating an absolute filter from a specific series of formulated materials in various configurations, variations, and compositions so as to permit the absolute filter to be cleaned or reused at an improved pressure condition of no less than 50% of its final, used pressure drop (e.g. initial pressure drop of 2 inches of water; used pressure drop of 12 inches of water; cleaned pressure drop of 2 to 6 inches of water). The herein disclosed improved absolute filter is so constructed that, by adding energy thereto in the form of mechanical impaction, ultrasonic or mechanical vibration, reverse air flow at pressures of 0.1 to 5 atmospheres, or vacuums from either the inlet or outlet sides of the filter at 0.5 atmospheres to 1 times 10-⁶ torr, the attractive force of the entrained particle will be neutralized and the particle will be released from the filter media.
In order to accomplish this, the filter media of the absolute filter must be chemically and physically modified to create a balance between the energy of the entrained particle and the filter media. If this is not done, the "added energy" of the cleaning processes noted above will cause the particle to release so as to damage the filter media, since excess energy will cause media rupture. The improved absolute filter, according to the disclosed invention, is formed by coating or impregnation of the media with the formulations listed in Table 1, Table 2, where the selected vehicles for the coatings are listed in Table 3. Some preferred formulations are found in Table 4.
What is especially to be noted is this, that the coating(s) alter the original or base filter media by strengthening, and this is established by an increase thereof in tensile strength, or percent of elongation, or abrasion resistance, or tear strength.
In summary the novel, improved absolute filter comprises a coating which comprises a polymeric binder with its appropriate solvent vehicle, and fillers and extenders which are added to the polymeric binder. These coatings add strength and toughness to the filter media, alter the volume resistivity of the media by making it more electrically conductive or protects the media from corrosive attack.
The teaching of this invention comprises the formation of the filter media from fibers, mattes, agglomerates, fiber overlays, woven fibers, entangled masses, reticulated structures, porous sinters or frits, at no greater than seventy percent theoretical density, which have been specially formulated with additives to cause the filter media to perform like the coated fibers priorly discussed and described.

Claims

1. An absolute, fluid filter, for removing components such as particulate materials, particulate-aerosol combinants, sublimated solids, and the like, from gas streams, said filter having a matrix formed from one material taken from a group of materials consisting of treated paper, glass, and mineral fibers, ceramic fibers, polymeric fibers, and cellulostic fibers, characterised by:

means coating said matrix material, and hardening, strengthening and toughening said matrix material, for enhancing components-release from said filter, whereby said filter is rendered cleanable and reusable.

2. A filter according to claim 1 characterised in that said coating means further comprises means for-altering the physical properties of the surface of said matrix material, to reduce impaction-retention of said components by said matrix material.

3. A filter according to claim 1 or 2 characterised in that said coating means further comprises means for reducing electrostatic attraction of said matrix material for components.

4. A filter according to claim 1, 2 or 3 characterised in that said coating means further comprises means for increasing diffusion capacity of said matrix material.

5. A filter according to any one of the preceding claims characterised in that said coating means further comprises means for imparting corrosion protection to said filter matrix.

,6. A filter according to any one of the preceding claims characterised in that said coating means comprises a filler agent; and said filler agent comprises means for gettering carcinogenic and like harmful vapors and/or particulates borne by said gas streams by altering said carcinogenic and like vapors and/or particulates chemical reactivity or changing the chemical structure thereof.

7. A filter according to claim 1 characterised in that said matrix material comprises a filter media formed of one of said glass, mineral, ceramic and polymeric fibers; and said coating means comprises a composition of graphitic powder and plastic.

8. A filter according to claim ? characterised in that said composition comprises a fiber-encasing film having a thickness within a range of approximately 10 to 1000 microinches.

9. A filter according to claim 1 characterised in that said matrix material comprises a filter medium formed of fibers which latter fibers are formed of a compound of a polymer and a.filler agent.

10. A filter according to claim 9 characterised in that said filler agent comprises carbon black and said polymer comprises polyethylene.

11. A filter according to claim 9 characterised in that said filler agent comprises a graphite intercalate, whereby said graphite intercalate defines means for gettering carcinogenic and like harmful vapors and/or particulates borne by said gas streams by altering said carcinogenic and like vapors and/or particulates chemical reactivity or changing the chemical structure thereof.

12. An absolute, fluid filter, for removing components such as particulate elements, particulate-aerosol combinants, sublimated solids, and the like, from gas streams, said filter having a matrix formed from one material taken fror a group of materials consisting of treated paper, glass, and mineral fibers, ceramic fibers, polymeric fibers, and cellulostic fibers, characterised by:

means mated to said matrix material, and hardening, strengthening and toughening said matrix material for facilitating release of said stream components from said filter, to render said filter easily and non-destructively cleanable whereby said filter, upon having been cleaned pursuant to a blinded condition thereof, manifests a fluid pressure thereacross of approximately fifty percent of such pressure as obtains in such filter in an original, unused condition.